The present invention relates more specifically to an arrangement for exchanging power in shunt connection, with a three-phase electric power network, said arrangement comprising: a Voltage Source Converter having three phase legs with each a series connection of switching cells in the form of so-called H-bridges comprising two switching elements connected in parallel and each having at least two semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a rectifying element connected in anti-parallel therewith, each said switching cell further comprising at least one energy storing capacitor connected in parallel with said switching elements, mid points between semiconductor assemblies of each switching element forming terminals of the switching cell for connection to corresponding terminals of adjacent switching cells for forming said series connection of switching cells, in which the three phase legs of the Voltage Source Converter are interconnected in a neutral point by forming a wye-connection, means configured to detect electrical conditions of said three-phase electric power network, and a control unit configured to control said semiconductor devices of said semiconductor assemblies of each switching cell and by that each switching cell dependent upon information received from said detecting means to deliver a voltage across the terminals of each said switching cell being zero, +U or −U, in which U is the voltage across said capacitor, for together with other switching cells of the phase leg deliver a voltage pulse being the sum of the voltages so delivered by each switching cell.
Such an arrangement is already known through for example U.S. Pat. No. 5,532,575 and the article “A Multilevel Voltage-Source inverter with Separate DC Sources for Static Var Generation”, 1995 IEEE, pages 2541-2548. An advantage of utilizing a Voltage Source Converter of this type in an arrangement for exchanging power with a three-phase electric power network is that already at a comparatively low number of such switching cells connected in series a comparatively high number of different levels of said voltage pulse delivered by the converter may be obtained, so that a said voltage with fundamental frequency having a shape being very close to a sinusoidal voltage may be obtained already without any smoothing filters. Furthermore, this may be obtained already by means of substantially lower switching frequencies than used in two or three level Voltage Source Converters. Furthermore, this makes it possible to obtain substantially lower losses and also reduces problems of filtering and harmonic currents and radio interferences, so that equipment therefor may be less costly. This altogether results in both a better performance of the arrangement and saving of costs with respect to such arrangements having converters with no energy storing capacitors built in into the switching cells.
It is of importance for reliable and efficient operation of such an arrangement that the energy stored in the capacitor of each switching cell of the converter is kept constant for keeping the direct voltage of each said cell constant. This implies that the wye-connected converter can in known arrangements of this type only generate reactive power, i.e. the current component is a positive-sequence current and is in quadrature to the voltage at the respective phase leg terminal of the converter. This is due to the fact that when a negative-sequence current is generated by a wye-connected converter and a positive-sequence voltage is assumed, then the energy in said capacitors will increase/decrease between phases resulting in direct voltage variations across the capacitors.
However, it may in some situations be very important to be able to create a negative-sequence current in order to move power from one of the phases of the power network to the other for obtaining balance of the power flow in a disturbed network or otherwise reducing disturbances in the network. In order to keep the energy of each said capacitor constant when generating a negative-sequence current it is then necessary to transfer energy between the phases of the converter. One method of doing this would consist of adding a zero-sequence voltage component to each phase leg leaving the potential of the neutral to that zero-sequence. However, this method requires a high number of switching cells due to the added voltage and present limit in the positive-sequence/negative-sequence current to be generated by the converter.